Longitudinally coupled resonator type surface acoustic wave filter and communication apparatus incorporating the same

ABSTRACT

A longitudinally coupled resonator type surface acoustic wave filter having a balance-unbalance conversion function achieves improved amplitude balance and phase balance. The surface acoustic wave filter includes first, second and third IDTs. The second IDT is positioned in the center of the three IDTs and has an even number of electrode fingers. The polarity of the electrode finger of the first IDT adjacent to the second IDT is opposite to the polarity of the electrode finger of the third IDT adjacent to the second IDT.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to longitudinally coupled resonatortype surface acoustic wave filters. More particularly, the presentinvention relates to longitudinally coupled resonator type surfaceacoustic wave filters having balance-unbalance conversion functions.

[0003] 2. Description of the Related Art

[0004] Recently, the sizes and weights of mobile phones have beengreatly reduced. With such a trend, the numbers and sizes of componentsused in mobile phones have also been reduced while making suchcomponents more and more multifunctional.

[0005] With the above-described considerations, various kinds of mobilephones have been provided. In these mobile phones, surface acoustic wavefilters are incorporated in the RF stages of the phones to havebalance-unbalance conversion functions, or so-called balun functions.

[0006]FIG. 22 is a schematic plan view for illustrating the electrodestructure of a conventional surface acoustic wave filter having abalance-unbalance conversion function.

[0007] In this filter, first, second and third IDTs 101, 102 and 103 arearranged in a surface acoustic wave propagating direction. Reflectors104 and 105 are arranged on each side of the surface acoustic wavepropagating direction in the region where the IDTs 101, 102 and 103 arearranged. When a wavelength determined by the electrode finger pitch ofeach of the IDTs 101, 102 and 103 is λI, both a distance between thecenters of the mutually adjacent electrode fingers of the IDTs 101 and102 and a distance between the centers of the mutually adjacentelectrode fingers of the IDTs 102 and 103 are 0.75 λI. The widths of theelectrode fingers 109 and 110 of the ends of the IDT 102 are increasedto reduce spaces between the IDTs. As a result, loss due to theirradiation of a bulk wave can be reduced. In FIG. 22, terminals 106 and107 are balanced signal terminals, and a terminal 108 is an unbalancedsignal terminal.

[0008] In such a surface acoustic wave filter having thebalance-unbalance conversion function, regarding propagationcharacteristics in a pass band between the unbalanced signal terminal108 and the balanced signal terminal 106 and between the unbalancedsignal terminal 108 and the balanced terminal 107, amplitudecharacteristics need to be equal and propagating signals need to be 180°out of phase with respect to each other. The condition in which theamplitude characteristics are equal is referred to as amplitude balanceand the degree at which the propagating signals are 180° out of phasewith respect to each other is referred to as phase balance.

[0009] The amplitude balance and the phase balance are defined asfollows when the surface acoustic wave filter having thebalance-unbalance conversion function is regarded as a device havingthree ports, such as the unbalanced input terminal as port 1 and thebalanced output terminals as ports 2 and 3.

[0010] Amplitude balance=|A|.

[0011] A=|20 log S21|−|20 log S31|.

[0012] Phase balance=|B−180|.

[0013] B=|∠S21−∠S31|.

[0014] In this definition, S21 denotes a transfer factor from the port 1to the port 2 and S31 denotes a transfer factor from the port 1 to theport 3.

[0015] Ideally, in the pass band of the filter, the amplitude balanceneeds to be 0 dB and the phase balance needs to be 0°. However, with theuse of the structure shown in FIG. 22, when intending to obtain a filterhaving a balance-unbalance conversion function, since the IDT 102 has anodd number of electrode fingers, the number of electrode fingersconnected to the balanced signal terminal 106 increases by one more thanthe number of electrode fingers connected to the balanced signalterminal 107. As a result, there is a problem in that the degree ofbalance is deteriorated. The higher the central frequency of the filteris, the more noticeable the deterioration. Thus, like DCS filters andPCS filters, in a filter having a central frequency near 1.9 GHz,sufficient balance cannot be obtained.

SUMMARY OF THE INVENTION

[0016] In order to overcome the problems described above, preferredembodiments of the present invention provide a longitudinally coupledresonator type surface acoustic wave filter having a balance-unbalanceconversion function, in which the amplitude balance and the phasebalance are greatly improved. In addition, preferred embodiments of thepresent invention provide a communication apparatus including such anovel longitudinally coupled resonator type surface acoustic wavefilter.

[0017] According to a preferred embodiment of the present invention, alongitudinally coupled resonator type surface acoustic wave filterhaving a balance-unbalance conversion function includes a piezoelectricsubstrate, first, second and third IDTs arranged on the piezoelectricsubstrate in a surface acoustic wave propagating direction, the secondIDT being positioned between the first and third IDTs and having an evennumber of electrode fingers.

[0018] According to another preferred embodiment of the presentinvention, a longitudinally coupled resonator type surface acoustic wavefilter having a balance-unbalance conversion function includesfirst-stage and second-stage longitudinally coupled resonator surfaceacoustic wave filters longitudinally coupled to each other, each of thefirst-stage and second-stage filters having a piezoelectric substrateand first, second and third IDTs arranged on the piezoelectric substratein a surface acoustic wave propagating direction, an unbalanced signalterminal connected to one end of the second IDT of the first-stagelongitudinally coupled resonator surface acoustic wave filter, a firstbalanced signal terminal connected to one end of the second IDT of thesecond-stage longitudinally coupled resonator surface acoustic wavefilter, a second balanced signal terminal connected to the other end ofthe second IDT of the second-stage filter, a first signal lineconnecting one end of the first IDT of the first-stage longitudinallycoupled resonator surface acoustic wave filter and one end of the firstIDT of the second-stage longitudinally coupled resonator surfaceacoustic wave filter, and a second signal line connecting one end of thethird IDT of the first-stage longitudinally coupled resonator surfaceacoustic wave filter and one end of the third IDT of the second-stagelongitudinally coupled resonator surface acoustic wave filter. In thisfilter, an electric signal propagating through the first signal line is180° out of phase with an electric signal propagating through the secondsignal line.

[0019] In the filter according to this preferred embodiment of thepresent invention, the second IDT of at least one of the first-stagefilter and the second-stage filter may have an even number of electrodefingers.

[0020] According to a third preferred embodiment of the presentinvention, a communication apparatus including the longitudinallycoupled resonator type surface acoustic wave filter according to theabove-described preferred embodiment of the present invention.

[0021] The above and other elements, characteristics, features, andadvantages of the present invention will be clear from the followingdetailed description of preferred embodiments of the present inventionin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 shows a schematic plan view for illustrating alongitudinally coupled resonator type surface acoustic wave filteraccording to a first preferred embodiment of the present invention;

[0023]FIG. 2 shows a schematic plan view for illustrating the electrodestructure of a longitudinally coupled resonator type surface acousticwave filter presented as a prerequisite for the first preferredembodiment of the present invention;

[0024]FIG. 3 shows a schematic plan view of a longitudinally coupledresonator type surface acoustic wave filter used as another prerequisitefor the first preferred embodiment of the present invention;

[0025]FIG. 4 shows a graph illustrating the relationship betweenamplitude balance and frequency characteristics of each of the firstpreferred embodiment of the present invention and a conventional device;

[0026]FIG. 5 shows a graph illustrating the relationship between phasebalance and frequency characteristics of each of the first preferredembodiment of the present invention and the conventional device;

[0027]FIG. 6 shows a schematic plan view for illustrating alongitudinally coupled resonator type surface acoustic wave filter as afirst modified example of the first preferred embodiment of the presentinvention;

[0028]FIG. 7 shows a schematic plan view for illustrating alongitudinally coupled resonator type surface acoustic wave filter as asecond modified example of the first preferred embodiment of the presentinvention;

[0029]FIG. 8 shows a schematic plan view for illustrating alongitudinally coupled resonator type surface acoustic wave filter as athird modified example of the first preferred embodiment of the presentinvention;

[0030]FIG. 9 shows a schematic plan view for illustrating alongitudinally coupled resonator type surface acoustic wave filter as afourth modified example of the first preferred embodiment of the presentinvention;

[0031]FIG. 10 shows a schematic plan view for illustrating alongitudinally coupled resonator type surface acoustic wave filter as afifth modified example of the first preferred embodiment of the presentinvention;

[0032]FIG. 11 shows a schematic plan view for illustrating alongitudinally coupled resonator type surface acoustic wave filteraccording to a second preferred embodiment of the present invention;

[0033]FIG. 12 shows a schematic plan view for illustrating a surfaceacoustic wave filter as a modified example of the second preferredembodiment of the present invention;

[0034]FIG. 13 shows a graph illustrating the relationship betweenamplitude balance and frequency characteristics of each of the secondpreferred embodiment and the conventional device;

[0035]FIG. 14 shows a graph illustrating the relationship between phasebalance and frequency characteristics of each of the second preferredembodiment and the conventional device;

[0036]FIG. 15 shows a schematic plan view for illustrating theconventional filter prepared for a comparison with the second preferredembodiment of the present invention;

[0037]FIG. 16 shows a schematic plan view for illustrating anothermodified example of the second preferred embodiment of the presentinvention;

[0038]FIG. 17 shows a schematic plan view for illustrating alongitudinally coupled resonator type surface acoustic wave filteraccording to a third preferred embodiment of the present invention;

[0039]FIG. 18 shows a graph illustrating the relationship betweenamplitude balance and frequency characteristics of each of the thirdpreferred embodiment of the present invention and the conventionaldevice;

[0040]FIG. 19 shows a graph illustrating the relationship between phasebalance and frequency characteristics of each of the third preferredembodiment of the present invention and the conventional device;

[0041]FIG. 20 shows a schematic plan view for illustrating alongitudinally coupled resonator type surface acoustic wave filteraccording to a fourth preferred embodiment of the present invention;

[0042]FIG. 21 shows a schematic block diagram for illustrating acommunication apparatus as an example of an apparatus including thelongitudinally coupled resonator type surface acoustic wave filteraccording to preferred embodiments of the present invention;

[0043]FIG. 22 shows a schematic plan view for illustrating theconventional longitudinally coupled resonator type surface acoustic wavefilter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0044] With reference to the drawings, the present invention will beclarified by describing preferred embodiments of a longitudinallycoupled resonator type surface acoustic wave filter according to thepresent invention.

[0045]FIG. 1 shows a schematic plan view for illustrating thelongitudinally coupled resonator type surface acoustic wave filter,preferably for use as a PCS reception filter, according to a firstpreferred embodiment of the present invention. In a longitudinallycoupled resonator type surface acoustic wave filter 200, there isprovided an electrode structure shown in FIG. 1 on a piezoelectricsubstrate 200A. A 40±5° Y-cut X-propagation LiTaO₃ substrate ispreferably used as the piezoelectric substrate 200A.

[0046] On the piezoelectric substrate, first, second and third IDTs 201,202 and 203 are arranged in a surface acoustic wave propagatingdirection. Reflectors 204 and 205 are arranged in the surface wavepropagating direction on the right and left of the region where the IDTs201, 202 and 203 are arranged. The IDTs 201, 202 and 203 and thereflectors 204 and 205 are preferably made of Al.

[0047] In other words, the longitudinally coupled resonator type surfaceacoustic wave filter 200 is a longitudinally coupled resonator typesurface acoustic wave filter having three IDTs.

[0048] In FIG. 1, in order to simplify the illustration, the number ofelectrodes shown in the figure is less than the number of electrodes tobe actually included in the device.

[0049] One end of each of the IDTs 201 and 203 is connected to anunbalanced signal terminal 212. One end of the IDT 202 is connected to abalanced signal terminal 210, and the other end thereof is connected toa balanced signal terminal 211.

[0050] The detailed design of an example of the present preferredembodiment of the longitudinally coupled resonator type surface acousticwave filter 200 will be presented below.

[0051] Electrode finger cross width W=78.8 λI.

[0052] The number of the electrode fingers of each of IDTs 201 and203=24.

[0053] The number of the electrode fingers of IDT 202=40.

[0054] IDT wavelength λI=2.03 μm.

[0055] The wavelength λR of each of reflectors 204 and 205=2.05 μm.

[0056] The number of the electrode fingers of each of the reflectors 204and 205=100.

[0057] IDT Gap=0.77 λI. In this case, the gap between the IDTs ispreferably substantially equivalent to a distance between the centers ofthe mutually adjacent electrode fingers of the IDTs.

[0058] IDT-reflector gap=0.55 λR. The gap between an IDT and a reflectoris preferably substantially equivalent to a distance between the centersof the mutually adjacent electrode fingers of the IDT and the reflector.

[0059] IDT Duty=0.60.

[0060] Reflector Duty=0.60.

[0061] Electrode-film thickness=0.08 λI.

[0062] In addition, as shown in FIG. 1, the widths of the electrodefingers 206 and 207 on each side of the IDT 202 are preferably broaderthan those of the remaining electrode fingers. With this arrangement,the space in the gap between the IDTs is reduced.

[0063] As the characteristics of the first preferred embodiment of thepresent invention, the total number of the electrode fingers of the IDT202 arranged in the center is preferably an even number and an electrodefinger 201 a of the IDT 201 adjacent to the IDT 202 is arranged todefine a signal electrode, whereas an electrode finger 203 a of the IDT203 adjacent to the IDT 202 is arranged to define a ground electrode. Asshown here, of the electrode fingers of the right IDT 201 and the leftIDT 203, the electrode fingers 201 a and 203 a adjacent to the centralsecond IDT 202 have opposite polarities. The reason for this will bedescribed below with reference to FIGS. 2 and 3.

[0064] In the conventional longitudinally coupled resonator type surfaceacoustic wave filter 100 shown in FIG. 22, when one of the electrodefingers of the central IDT 102 is removed, as shown in FIG. 2, thenumber of the electrode fingers of the central IDT 102A is an evennumber. On the other hand, since a distance A between the IDT 102A andthe IDT 103 is increased by about 0.5 λI, loss due to the irradiation ofa bulk wave is increased.

[0065] Thus, as shown in FIG. 3, there can be considered a structure inwhich the third IDT 103 is shifted by about 0.5 λI toward the IDT 102A.However, in the structure shown in FIG. 3, the IDT 101 and the IDT 103are 180° out of phase with each other.

[0066] Therefore, in this preferred embodiment of the present invention,as shown in FIG. 1, by having the IDT 201 reversed from the IDT 203, theIDT 201 and the IDT 203 are in phase with each other.

[0067]FIG. 4 shows a graph illustrating the relationship betweenamplitude balance and frequencies in the longitudinally coupledresonator type surface acoustic wave filter according to the firstpreferred embodiment of the present invention. FIG. 5 shows a graphillustrating the relationship between phase balance and frequencies inthe same filter. In FIGS. 4 and 5, solid lines show the results of thefirst preferred embodiment of the present invention. In addition, forcomparison, in FIGS. 4 and 5, broken lines show the characteristics ofthe conventional filter shown in FIG. 22.

[0068] In this case, the conventional filter has the same detaileddesign as that of the first preferred embodiment, except that one of theelectrode fingers of the central IDT in the conventional filter isremoved.

[0069] A pass-band frequency range used in the PCS reception filter isbetween about 1930 MHz to about 1990 MHz. As seen in FIG. 4, in thefrequency band range, the maximum amplitude balance is 3.2 dB in theconventional filter, whereas it is about 2.7 dB in the first preferredembodiment of the present invention. That is, the amplitude balance isimproved by about 0.5 dB in this preferred embodiment of the presentinvention. Moreover, as shown in FIG. 5, whereas the maximum phasebalance in the conventional filter is 21°, the maximum phase balance inthe first preferred embodiment of the present invention is about 17°.That is, obviously, the phase balance is improved by about 4° in thispreferred embodiment of the present invention.

[0070] As mentioned above, in this preferred embodiment of the presentinvention, the total number of the electrode fingers of the centralsecond IDT 202 is an even number and the polarity of the electrodefinger of the first IDT 201 adjacent to the central second IDT 202 isreversed from the polarity of the electrode finger of the third IDT 203adjacent to the IDT 202. Thus, as compared with the conventional filter,obviously, the first preferred embodiment of the present inventionprovides a longitudinally coupled resonator type surface acoustic wavefilter having a balance-unbalance conversion function, in which theamplitude balance and the phase balance are both greatly improved.

[0071] This preferred embodiment preferably uses a 40±5° Y-cutX-propagation LiTaO₃ substrate. However, the substrate used in thepresent invention is not restricted to the LiTaO₃ substrate. Any otherpiezoelectric substrate may be used such as a 64 to 72° Y-cutX-propagation LiNbO₃ substrate or a 41° Y-cut X-propagation LiNbO₃substrate, for example. Other suitable substrates may also be used.

[0072] In addition, in the first preferred embodiment of the presentinvention, a balanced signal is obtained from the central second IDT.However, as shown in FIG. 6, the balanced signal may be obtained fromeach of the first IDT 201 and the third IDT 203 on each side of thecentral IDT 202. In FIG. 6, terminals 213 and 214 are balanced signalterminals connected to the first and third IDTs 201 and 203,respectively, and a terminal 215 is an unbalanced signal terminalconnected to the central second IDT 202.

[0073]FIG. 7 shows a schematic plan view for illustrating the electrodestructure of another modified example of the first preferred embodimentof the present invention. As shown in FIG. 7, a surface acoustic waveresonator 216 may be connected between the first and third IDTs 201 and203 and a terminal 212.

[0074]FIG. 8 shows a schematic plan view for illustrating the electrodestructure of another modified example of the first preferred embodimentof the present invention. In a longitudinally coupled resonator typesurface acoustic wave filter 217 shown in FIG. 8, the longitudinallycoupled resonator type surface acoustic wave filters 200 according tothe first preferred embodiment are longitudinally connected in atwo-stage structure.

[0075] In addition, FIG. 9 shows a schematic plan view for illustratinga longitudinally coupled resonator type surface acoustic wave filter asanother modified example of the first preferred embodiment of thepresent invention. In FIG. 9, the longitudinally coupled resonator typesurface acoustic wave filter 200 of the first preferred embodiment islongitudinally connected to a 3-IDT-type longitudinally coupledresonator type surface acoustic wave filter 219 including the centralIDT 218 having an odd number of electrode fingers. In other words, inthe multi-stage longitudinally coupled resonator type surface acousticwave filters, even when at least only one-stage filter is defined by thelongitudinally coupled resonator type surface acoustic wave filter 200of the first preferred embodiment, as in the first preferred embodiment,greatly improved amplitude and phase balances are achieved.

[0076]FIG. 10 shows a schematic plan view for illustrating the electrodestructure of another modified example of the longitudinally coupledresonator type surface acoustic wave filter of the first preferredembodiment. In a longitudinally coupled resonator type surface acousticwave filter 220 shown in FIG. 10, narrow pitch electrode finger sectionsN1 to N4 are disposed in first, second and third IDTs 221, 222 and 223.Specifically, in the IDT 221, the narrow pitch electrode finger sectionN1, is arranged such that the electrode finger pitches of some portionsfrom the end portion of the IDT 222 are narrower than the electrodefinger pitches of the remaining portions of the IDT 221. Similarly, oneach end portion of the IDT 222, the narrow pitch electrode fingersections N2 and N3 are provided. In addition, in the IDT 223, on theside adjacent to the IDT 222, the narrow pitch electrode finger sectionN4 is provided. As shown here, in the regions where the IDTs areadjacent to each other, even when disposing the narrow pitch electrodefinger sections having relatively narrow electrode finger pitches, byarranging the remaining portions like the first preferred embodiment,greatly improved amplitude and phase balances are achieved.

[0077]FIG. 11 shows a schematic plan view for illustrating the electrodestructure of a longitudinally coupled resonator type surface acousticwave filter according to a second preferred embodiment of the presentinvention.

[0078] The second preferred embodiment of the present invention relatesto an AMPS reception filter.

[0079] Similar to the first preferred embodiment, the electrodestructure shown in FIG. 11 is preferably formed on a 40±5° Y-cutX-propagation LiTaO₃ substrate to constitute a longitudinally coupledresonator type surface acoustic wave filter 300 of the second preferredembodiment of the present invention.

[0080] In the longitudinally coupled resonator type surface acousticwave filter 300, a first longitudinally coupled resonator type surfaceacoustic wave filter 301 and a second longitudinally coupled resonatortype surface acoustic wave filter 302 are longitudinally connected toeach other. The first and second longitudinally coupled resonator typesurface acoustic wave filters 301 and 302 preferably have the samestructure.

[0081] Similar to the first preferred embodiment, the first-stagelongitudinally coupled resonator type surface acoustic wave filter 301includes first, second and third IDTs 303, 304 and 305 arranged in asurface acoustic wave propagating direction and reflectors 306 and 307,and the second-stage longitudinally coupled resonator type surfaceacoustic wave filter 302 includes first, second and third IDTs 308, 309and 310 in the surface acoustic wave propagating direction andreflectors 311 and 312. The reflectors 306, 307, 311, and 312 aredisposed on the right and left sides of the regions where the IDTs ofthe filters 301 and 302 are arranged. One end of each of the first andthird IDTs 303 and 305 of the first longitudinally coupled resonatortype surface acoustic wave filter 301 is connected to one end of each ofthe first and third IDTs 308 and 310 of the second longitudinallycoupled resonator type surface acoustic wave filter 302 via first andsecond signal lines 316 and 317. One end of the IDT 304 is connected toa terminal 313. One end of the IDT 309 is connected to a terminal 314and the other end thereof is connected to a terminal 315. The remainingends of the IDTs 303, 304 and 305 and 308, 309 and 310 are connected toground potentials.

[0082] The terminals 314 and 315 are balanced signal terminals. Theterminal 313 is an unbalanced signal terminal. The detailed design of anexample of the filter 300 will be presented below.

[0083] Electrode finger cross width W=49.0 λI.

[0084] The number of the electrode fingers of first IDTs 303 and 308=24.

[0085] The number of the electrode fingers of second IDTs 304 and309=34.

[0086] The number of the electrode fingers of third IDTs 305 and 310=25.

[0087] IDT wavelength λI=4.49 μm.

[0088] Reflector wavelength λR=4.64 μm.

[0089] The number of reflector electrode fingers=120.

[0090] IDT gap=0.79 λI.

[0091] IDT-reflector gap=0.47 λR.

[0092] IDT duty=0.73.

[0093] Reflector duty=0.55.

[0094] Electrode film thickness=0.08 λI.

[0095] In this preferred embodiment of the present invention, the widthsof the electrode fingers 304 a, 304 b, 309 a, and 309 b on each end ofthe central second IDTs 304 and 309 are broadened to reduce spacesbetween the adjacent IDTs.

[0096] In this preferred embodiment, the number of the electrode fingersof each of the IDTs 304 and 309 is preferably an even number as in thecase of the first preferred embodiment. In addition, the electrodefingers 303 a and 305 a of the IDTs 303 and 305 adjacent to the IDT 304and the electrode fingers 308 a and 310 a of the IDTs 308 and 310adjacent to the IDT 309 are arranged to define ground electrodes.

[0097] In the first longitudinally coupled resonator type surfaceacoustic wave filter 301, the IDTS 303 and 305 are 180° out of phasewith the IDT 304. Thus, when the IDTs 303 and 305 are connected inparallel to make the single stage filter, it is impossible to obtainfilter characteristics. However, simultaneously, in the secondlongitudinally coupled resonator type surface acoustic wave filter 302,the IDTs 308 and 310 are also 180° out of phase with the IDT 309. As aresult, since surface waves propagating from the IDTs 308 and 310 to theIDT 309 are in phase with each other, filter characteristics can beobtained by connecting the first and second longitudinally coupledresonator type surface acoustic wave filters 301 and 302. In otherwords, an electric signal propagating through a first signal line 316 is180° out of phase with an electric signal propagating through a secondsignal line 317.

[0098]FIG. 13 shows the relationship between amplitude balance andfrequencies in the second preferred embodiment of the present invention.FIG. 14 shows the relationship between phase balance and frequencies. InFIGS. 13 and 14, solid lines indicate the results of the secondpreferred embodiment and broken lines indicate the results of aconventional device shown in FIG. 15, which is prepared for comparison.

[0099] In a conventional longitudinally coupled resonator type surfaceacoustic wave filter 401 shown in FIG. 15, the numbers of the electrodefingers of first, second and third IDTs 402, 403 and 404 are preferablythe same as those of the second preferred embodiment, except that thenumber of the electrode fingers of the first IDT is preferably 24, thenumber of the electrode fingers of the second IDT is preferably 35, andthe number of the electrode fingers of the third IDT is preferably 24.

[0100] A pass-band frequency range in the AMPS reception filter isbetween approximately 860 MHz and approximately 894 MHz.

[0101] In the frequency range, the maximum amplitude balance is 1.9 dBin the conventional filter, whereas it is about 0.9 dB in the secondpreferred embodiment of the present invention. Thus, obviously, theamplitude balance is improved by about 1.0 dB in the second preferredembodiment.

[0102] In addition, whereas the maximum phase balance is 17° in theconventional filter, it is about 8° in the second preferred embodiment.The phase balance is improved by about 9° in the second preferredembodiment of the present invention.

[0103] As mentioned above, the reason that the amplitude and phasebalances are improved is as follows. The second IDT has an even numberof electrode fingers and the electric signal propagating through thefirst signal line 316 is 180° out of phase with the electric signalpropagating through the second signal line 317. As a result, theelectrode fingers of the first and third IDTs adjacent to the centralsecond IDT can be both arranged to define ground electrodes, and adistance B (see FIG. 11) between the electrode finger 309 a connected tothe terminal 315 and the electrode signal finger 308 b of the IDT 308 issubstantially equal to a distance C (see FIG. 11) between the electrodefinger 309 b connected to the terminal 314 and the signal electrodefinger 310 b of the IDT 310. In contrast, in the conventional deviceshown in FIG. 15, distances D and E between electrode fingers, specifiedin the figure, between the adjacent IDTs in the second longitudinallycoupled resonator type surface acoustic wave filter are differentiatedby about 0.5 λI.

[0104] In the second preferred embodiment, the electrode fingers of theright first IDT and the left third IDT adjacent to the second IDT areground electrodes. However, even when these electrode fingers constitutesignal electrodes, the same advantages can be obtained.

[0105] As mentioned above, in the second preferred embodiment, the firstand second longitudinally coupled resonator type surface acoustic wavefilters 301 and 302 including the second IDTs having the even number ofelectrode fingers are longitudinally connected in the two-stagestructure. Additionally, the electric signal propagating through thefirst signal line 316 is 180° out of phase with the electric signalpropagating through the second signal line 317. With this arrangement,the polarity of the electrode fingers of the central second IDT can bethe same as the polarities of the electrode fingers of the first andthird IDTs adjacent to the second IDT. Both of the amplitude balance andthe phase balance are also greatly improved.

[0106] Also, in the case where narrow pitch electrode fingers are usedat the portion where two IDTs are adjacent to each other in thestructure shown in FIG. 11, with respect to the second preferredembodiment, as shown in FIG. 10, the characteristics which indicatesimproved balance can be obtained.

[0107]FIG. 12 shows a schematic plan view for illustrating a modifiedexample of the second preferred embodiment of the present invention.This modified example is differentiated from the second preferredembodiment in that, in the arrangement shown in FIG. 11, the first andsecond surface acoustic wave filters 301 and 302 are symmetrical withrespect to the axis Z in FIG. 12. However, in the modified example shownin FIG. 12, the surface acoustic wave filters 302 and 302 aresymmetrical with respect to the point Y in the figure, that is, thecenter of the entire electrode structure including the surface acousticwave filters 301 and 302.

[0108]FIG. 16 shows a schematic plan view for illustrating anotherexample of the second preferred embodiment of the present invention. Inthis modified example, first and second longitudinally coupled resonatortype surface acoustic wave filters 301 and 321 are longitudinallyconnected to each other. Unlike the second preferred embodiment, in thesecond-stage longitudinally coupled resonator type surface acoustic wavefilter 321, the central second IDT 322 is preferably split into twoparts. The remaining parts are preferably substantially the same asthose in the second preferred embodiment. Thus, the same referencenumerals as those used in the second embodiment are given to the sameparts in the modified example, and the explanation thereof will beomitted.

[0109] The second IDT 322 has an interdigital electrode 322 a, and twointerdigital electrodes 322 b and 322 c arranged such that the electrodefingers of the interdigital electrode 322 a are interdigitated with theelectrode fingers of the interdigital electrodes 322 b and 322 c. Inother words, one of a pair of the interdigital electrodes defining thesecond IDT 322 is split into the two interdigital electrodes 322 b and322 c, which are connected to balanced signal terminals 314 and 315. Inthis case, in the first-stage longitudinally coupled resonator typesurface acoustic wave filter 301, the IDT 303 is 180° out of phase withthe IDT 305. In the second-stage longitudinally coupled resonator typesurface acoustic wave filter 321, the first IDT 308 is 180° out of phasewith the second IDT 310. With this arrangement, an electric signalpropagating through a signal line 316 is 180° out of phase with anelectric signal propagating through a signal line 317. Thus, thepolarities of the electrode fingers of the IDT 308 and the IDT 310adjacent to the IDT 322 are the same. As a result, the amplitude andphase balances in this filter are greatly improved as with the secondpreferred embodiment of the present invention.

[0110]FIG. 17 shows a schematic plan view for illustrating the electrodestructure of a longitudinally coupled resonator type surface acousticwave filter according to a fourth preferred embodiment of the presentinvention.

[0111] A longitudinally coupled resonator type surface acoustic wavefilter 600 of the fourth preferred embodiment preferably has the samestructure as that of the second preferred embodiment, except for thefollowing points.

[0112] In this preferred embodiment, in first-stage and second-stagelongitudinally coupled resonator type surface acoustic wave filters 601and 602, the numbers of the electrode fingers of first, second and thirdIDTs are determined, in which the number of the electrode fingers ofeach of the first IDTs 603 and 608 is preferably 24, the number of theelectrode fingers of each of the second IDTs 604 and 609 is preferably35, and the number of the electrode fingers of each of the third IDTs605 and 610 is preferably 24.

[0113] In the fourth preferred embodiment, characteristically, thepolarity of an electrode finger 603 a of the first IDT 603 adjacent tothe IDT 604 is opposite to the polarity of an electrode finger 605 a ofthe third IDT 605 adjacent to the IDT 604. In addition, the polarity ofan electrode finger 608 a of the first IDT 608 adjacent to the IDT 609is opposite to an electrode finger 610 a of the third IDT 610 adjacentto the IDT 609.

[0114] When considering only the first-stage longitudinally coupledresonator type surface acoustic wave filter 601, each of the IDTs 603and 605 is 180° out of phase with the IDT 604. Thus, it is impossible toobtain filter characteristics only by the single-stage filter structurein which the IDTs 603 and 605 are connected in parallel to each other.However, simultaneously, in the second-stage longitudinally coupledresonator type surface acoustic wave filter 602, each of the IDTs 608and 610 is 180° out of phase with the IDT 609. As a result, sincesurface acoustic waves propagating from the IDTs 608 and 610 to the IDT609 are in phase with each other, when the first-stage filter and thesecond-stage filter are connected, filter characteristics can beobtained.

[0115] A terminal 615 is an unbalanced signal terminal and terminals 616and 617 are balanced signal terminals.

[0116] In FIG. 18, a solid line shows the relationship between amplitudebalance and frequency characteristics of the longitudinally coupledresonator type surface acoustic wave filter of the fourth preferredembodiment. In FIG. 19, a solid line shows the relationship betweenphase balance and frequency characteristics of the surface acoustic wavefilter. For comparison, broken lines show the result of the conventionalfilter shown in FIG. 15.

[0117] As found in FIG. 19, the maximum amplitude balance in a pass bandin the AMPS reception filter is 1.9 dB in the conventional filter,whereas it is about 1.2 dB in the fourth preferred embodiment of thepresent invention. Thus, the amplitude balance is improved by about 0.7dB. Additionally, as shown in FIG. 19, whereas the maximum phase balanceis 17° in the conventional filter, it is about 11° in the fourthpreferred embodiment. Thus, the phase balance is improved by about 6° inthe fourth preferred embodiment.

[0118] In other words, in the fourth preferred embodiment, thefirst-stage and first-stage longitudinally coupled resonator typesurface acoustic wave filters having three IDTs are longitudinallyconnected to each other. In addition, the polarities of the electrodefingers of the first and third IDTs adjacent to the central second IDTare opposite to each other. With this arrangement, the amplitude balanceand the phase balance are greatly improved.

[0119]FIG. 20 shows a schematic plan view for illustrating the electrodestructure of the longitudinally coupled resonator type surface acousticwave filter according to a fifth preferred embodiment of the presentinvention. The fifth preferred embodiment uses 3-IDT-type longitudinallycoupled resonator type surface acoustic wave filters 901, 902, 903 and904 having first, second and third IDTs. In each of the longitudinallycoupled resonator type surface acoustic wave filters 901, 902, 903 and904, the central second IDT has even electrode fingers, and thepolarities of the electrode fingers of the right and left IDTs adjacentto the central second IDT are the same. For example, in thelongitudinally coupled resonator type surface acoustic wave filter 901,the electrode finger 907 a of the first IDT 907 and the electrode finger909 a of the third IDT 909 adjacent to the second IDT 908 have the samepolarity.

[0120] Although the longitudinally coupled resonator type surfaceacoustic wave filters 901, 902 and 903 preferably have the samestructure, only the longitudinally coupled resonator type surfaceacoustic wave filter 902 has the second IDT 910 reversed.

[0121] The reference numeral 911 denotes an unbalanced signal terminaland the reference numerals 912 and 913 denote balanced signal terminals.

[0122] In each of the first, second, third and fourth longitudinallycoupled resonator type surface acoustic wave filters 901, 902, 903 and904, the first IDT is 180° out of phase with the third IDT. For example,in the longitudinally coupled resonator type surface acoustic wavefilter 901, the first IDT 907 is 180° out of phase with the third IDT909.

[0123] In the fifth preferred embodiment, similar to the second, thirdand fourth preferred embodiments, the amplitude balance and the phasebalance in the filter are greatly improved. Moreover, in the fifthpreferred embodiment, output impedance substantially quadruples.

[0124] FIGS. 21 shows a schematic plan view for illustrating acommunication apparatus 60 incorporating the longitudinally coupledresonator type surface acoustic wave filter according to other preferredembodiments of the present invention.

[0125] In FIG. 21, an antenna 61 is connected to a duplexer 62. Asurface acoustic wave filter 64 and an amplifier 65 are connectedbetween the duplexer 62 and a mixer 63 of the reception section. Anamplifier 67 and a surface acoustic wave filter 68 are connected betweenthe duplexer 62 and a mixer 66 of the transmission section. As shownhere, when the amplifier 65 is adapted to a balanced signal, thelongitudinally coupled resonator type surface acoustic wave filter ofvarious preferred embodiments of the present invention can be suitablyused as the surface acoustic wave filter 64.

[0126] As described above, in the longitudinally coupled resonator typesurface acoustic wave filter according to various preferred embodimentsof the present invention, the central second IDT preferably has an evennumber of electrode fingers. In addition, the polarities of electrodefingers adjacent to the second IDT in the first and third IDTs disposedon the right-and-left sides of the second IDT are opposite. Thus, theamplitude balance and the phase balance between the balanced signalterminals are greatly effectively improved. As a result, preferredembodiments of the present invention provide a surface acoustic wavefilter having an excellent balance-unbalance conversion function.

[0127] Furthermore, in the longitudinally coupled resonator type surfaceacoustic wave filter according to other preferred embodiments of thepresent invention, first-stage and second-stage longitudinally coupledresonator type surface acoustic wave filters having first, second andthird IDTs are longitudinally connected to each other. One end of thesecond IDT of the first-stage longitudinally coupled resonator typesurface acoustic wave filter is connected to an unbalanced signalterminal. Additionally, both ends of the second IDT of the second-stagelongitudinally coupled resonator type surface acoustic wave filter areconnected to a pair of balanced signal terminals. In this arrangement,an electric signal propagating through a first signal line connectingone end of the first IDT of the first-stage longitudinally coupledresonator type surface acoustic wave filter and one end of the first IDTof the second-stage longitudinally coupled resonator type surfaceacoustic wave filter is 180° out of phase with an electric signalpropagating through a second signal line connecting one end of the thirdIDT of the first-stage longitudinally coupled resonator type surfaceacoustic wave filter and one end of the third IDT of the second-stagelongitudinally coupled resonator type surface acoustic wave filter. As aresult, the polarity of the electrode finger of the second IDT adjacentto each of the first and third IDTs can be the same as the polarity ofthe electrode finger of each of the first and third IDTs adjacent to thesecond IDT. Thus, the amplitude balance and the phase balance in thefilter are effectively improved.

[0128] According to this preferred embodiment of the present invention,in at least one of the first-stage and second-stage longitudinallycoupled resonator type surface acoustic wave filters, when the secondIDT has an even number of electrode fingers, the amplitude balance andthe phase balance are improved even more.

[0129] In various preferred embodiments of the present invention, thelongitudinally coupled resonator type surface acoustic wave filter hasthe above-described balance-unbalance conversion function and thebalance between the pair of balanced signal terminals is greatlyimproved. Accordingly, when producing a communication apparatusincorporating the longitudinally coupled resonator type surface acousticwave filter, the characteristics of the communication apparatus can begreatly improved while reducing the size of the apparatus.

[0130] While the present invention has been described with reference towhat are at present considered to be preferred embodiments, it is to beunderstood that various changes and modifications may be made theretowithout departing from the invention in its broader aspects andtherefore, it is intended that the appended claims cover all suchchanges and modifications as fall within the true spirit and scope ofthe invention.

What is claimed is:
 1. A longitudinally coupled resonator type surfaceacoustic wave filter having a balance-unbalance conversion function, thefilter comprising: a piezoelectric substrate; and first, second andthird IDTs arranged on the piezoelectric substrate in a surface acousticwave propagating direction, the second IDT being located between thefirst and third IDTS and having an even number of electrode fingers. 2.A longitudinally coupled resonator type surface acoustic wave filteraccording to claim 1 , wherein the piezoelectric substrate is made ofone of LiTaO₃ and LiNbO₃.
 3. A longitudinally coupled resonator typesurface acoustic wave filter according to claim 1 , further comprisingreflectors arranged in the surface wave propagating direction on theright and left of the region where the first, second and third IDTs arearranged.
 4. A longitudinally coupled resonator type surface acousticwave filter according to claim 1 , wherein widths of the electrodefingers on each side of the second IDT are larger than those of theremaining electrode fingers.
 5. A longitudinally coupled resonator typesurface acoustic wave filter according to claim 1 , wherein theelectrode fingers adjacent to the second IDT have opposite polarities.6. A longitudinally coupled resonator type surface acoustic wave filteraccording to claim 1 , further comprising a surface acoustic waveresonator connected between the first and third IDTs and a terminal. 7.A longitudinally coupled resonator type surface acoustic wave filteraccording to claim 1 , wherein each of the first, second and third IDTsinclude narrow pitch electrode finger sections that are relativelynarrower than others of the electrode finger sections included in thefirst, second and third IDTs.
 8. A communication apparatus comprisingthe longitudinally coupled resonator type surface acoustic wave filteraccording to claim 1 .
 9. A longitudinally coupled resonator typesurface acoustic wave filter having a balance-unbalance conversionfunction, the filter comprising: first-stage and second-stagelongitudinally coupled resonator type surface acoustic wave filterslongitudinally coupled to each other, each of the first-stagelongitudinally coupled resonator type surface acoustic wave and thesecond-stage longitudinally coupled resonator type surface acoustic wavefilter including a piezoelectric substrate and first, second and thirdIDTs arranged on the piezoelectric substrate in a surface acoustic wavepropagating direction; an unbalanced signal terminal connected to oneend of the second IDT of the first-stage longitudinally coupledresonator type surface acoustic wave filter; a first balanced signalterminal connected to one end of the second IDT of the second-stagelongitudinally coupled resonator type surface acoustic wave filter; asecond balanced signal terminal connected to the other end of the secondIDT of the second-stage longitudinally coupled resonator type surfaceacoustic wave filter; a first signal line connecting one end of thefirst IDT of the first-stage longitudinally coupled resonator typesurface acoustic wave filter and one end of the first IDT of thesecond-stage longitudinally coupled resonator type surface acoustic wavefilter; and a second signal line connecting one end of the third IDT ofthe first-stage longitudinally coupled resonator type surface acousticwave filter and one end of the third IDT of the second-stagelongitudinally coupled resonator type surface acoustic wave filter;wherein an electric signal propagating through the first signal line is180° out of phase with an electric signal propagating through the secondsignal line.
 10. A longitudinally coupled resonator type surfaceacoustic wave filter according to claim 9 , wherein the second IDT of atleast one of the first-stage longitudinally coupled resonator typesurface acoustic wave filter and the second-stage longitudinally coupledresonator type surface acoustic wave filter has an even number ofelectrode fingers.
 11. A longitudinally coupled resonator type surfaceacoustic wave filter according to claim 9 , wherein the piezoelectricsubstrate of each of the first-stage and second-stage longitudinallycoupled resonator type surface acoustic wave filters is made of one ofLiTaO₃ and LiNbO₃.
 12. A longitudinally coupled resonator type surfaceacoustic wave filter according to claim 9 , wherein each of first-stageand second-stage longitudinally coupled resonator type surface acousticwave filters further comprises reflectors arranged in the surface wavepropagating direction on the right and left of the region where thefirst, second and third IDTs are arranged.
 13. A longitudinally coupledresonator type surface acoustic wave filter according to claim 9 ,wherein in each of the first-stage and second-stage longitudinallycoupled resonator type surface acoustic wave filters, widths of theelectrode fingers on each side of the second IDT are larger than thoseof the remaining electrode fingers.
 14. A longitudinally coupledresonator type surface acoustic wave filter according to claim 9 ,wherein in each of the first-stage and second-stage longitudinallycoupled resonator type surface acoustic wave filters, the electrodefingers adjacent to the second IDT have opposite polarities.
 15. Alongitudinally coupled resonator type surface acoustic wave filteraccording to claim 9 , wherein each of the first-stage and second-stagelongitudinally coupled resonator type surface acoustic wave filtersfurther comprises a surface acoustic wave resonator connected betweenthe first and third IDTs and a terminal.
 16. A longitudinally coupledresonator type surface acoustic wave filter according to claim 9 ,wherein in each of the first-stage and second-stage longitudinallycoupled resonator type surface acoustic wave filters, each of the first,second and third IDTs include narrow pitch electrode finger sectionsthat are relatively narrower than others of the electrode fingersections included in the first, second and third IDTs.
 17. Alongitudinally coupled resonator type surface acoustic wave filteraccording to claim 9 , wherein the first-stage and second-stagelongitudinally coupled resonator type surface acoustic wave filters havethe same structure.
 18. A longitudinally coupled resonator type surfaceacoustic wave filter according to claim 9 , wherein in each of thefirst-stage and second-stage longitudinally coupled resonator typesurface acoustic wave filters, the finger electrodes of the first andthird IDTs that are adjacent to the central second IDT are arranged todefine ground electrodes.
 19. A longitudinally coupled resonator typesurface acoustic wave filter according to claim 9 , wherein in each ofthe first-stage and second-stage longitudinally coupled resonator typesurface acoustic wave filters, the polarity of the electrode fingers ofthe second IDT is the same as the polarities of the electrode fingers ofthe first and third IDTs adjacent to the second IDT.
 20. Alongitudinally coupled resonator type surface acoustic wave filteraccording to claim 9 , wherein the first-stage and second-stagelongitudinally coupled resonator type surface acoustic wave filters aresymmetrical to each other.
 21. A longitudinally coupled resonator typesurface acoustic wave filter according to claim 9 , wherein in each ofthe first-stage and second-stage longitudinally coupled resonator typesurface acoustic wave filters, the second IDT is split into two parts.22. A communication apparatus comprising the longitudinally coupledresonator type surface acoustic wave filter according to claim 9 .